Mendacity is a tendency to lie. Your friend might swear that he didn't eat your secret chocolate stash, but you'll find it hard to believe him if he's known for his mendacity.
Anyone in the habit of lying frequently has the characteristic of mendacity. People often accuse government officials of mendacity, or being less than honest. You're bound to get frustrated by the mendacity of your friend who's a pathological liar. Mendacity comes from the Latin root word mendacium, or "lie." Don't confuse mendacity with a similar-sounding word, audacity — which means "fearlessness, daring, or bravery."